Embryonic chick duodenum, maintained in organ culture in a defined medium responds to vitamin D-like steroids in vitro similarly to the intact chick intestine. Chronologically, the responses are: elevated cAMP concentration, increased RNA synthesis, de novo induction of a specific calcium-binding protein (CaBP) and, concomitantly, increased transmucosal calcium transport, elevated alkaline phosphatase activity and, finally, increased phosphate transport. CaBP biosynthesis is regulated by cAMP and Ca 2 ion. While a number of hormones, including PTH and CT, have no effect on vitamin D-mediated responses, glucocorticoids and triiodothyronine (and zinc) augment CaBP synthesis. Insulin interacts with glucocorticoids in maintaining tissue weight during culture. An important, if not singular, role of CaBP in diffusional permeability of calcium is in the stimulation of calcium influx at the mucosal surface. Recent work has demonstrated the induction of metallothionein(s) by Cd 2 ion or Zn 2 ion in the culture medium even while 1 alpha, 25 (OH)2D3-induced CaBP biosynthesis was inhibited. Metallothionein may be important in zinc transport. Other work has related the CaBP-inducing biopotency of numerous vitamin D metabolites and analogs. Analogs fluorinated in the 1 alpha position were shown to be considerably more potent than non-fluorinated counterparts. The most potent vitamin D steroid yet tested in this system is 24,24-F2-1alpha,25(OH)2D3 which is more potent than the naturally occurring metabolite, 1 alpha,25(OH)2D3. Work in progress includes (1) continued development of an "optimal" culture medium by addition of hormones, "growth factors" and other agents to the culture medium; (2) continued study of vitamin D- or anti-vitamin D- analog biopotency in CaBP induction; (3) continued analysis of the interaction of glucocorticoids and 1 alpha, 25(OH)2D3 in CaBP induction, particularly, measurement of 1 alpha,25 (OH)2D3-receptor levels and CaBP-mRNA levels in the cultured duodenum; and (4) continued study of the mode of action of Cd 2 ion in the inhibition of CaBP synthesis and metallothionein induction. As always, in vitro data will be compared and/or related to in vivo data to assess the physiological significance of experimental observations.